Hindered phenolic esters of oligomeric glycols as chain terminators for polyvinyl chloride polymerization

ABSTRACT

An improved process for terminating the suspension polymerization reaction of vinyl chloride monomer into polyvinyl chloride which comprises adding hindered phenols of the structure presented hereinafter to the polymerization reaction at a point in time when a predetermined amount of monomer conversion has occurred.

This application is a continuation of application Ser. No. 280,389,filed 7/6/81, now abandoned.

The basic process of vinyl polymerization involves the addition of avinyl monomer to a growing polymer chain. The polymerization isconducted at the proper temperature under pressure in the presence of aninitiator. The polymer chain continues to grow in length until it isterminated by some means.

With regard to vinyl chloride polymerization, the overwhelming amount ofpolyvinyl chloride is prepared by suspension polymerization. In thisprocess, the vinyl chloride monomer and an initiator are dispersed byagitation into a water phase at proper temperature and pressure.Suspending agents such as methyl or ethyl cellulose, gelatin, polyvinylalcohol or other water-soluble polymers are utilized to stabilize thesuspension during the polymerization reaction. The fine granules ofpolymer in the form of a slurry are discharged from the reactor andcentrifuged or filtered to remove the water. Thorough washing and dryingof the polymer to remove traces of the suspension stabilizer and thereaction medium conclude the procedure.

Toward the end of the polymerization cycle the pressure in the systembegins to drop, followed very shortly by a peak in the polymerizationrate. Beyond the peak, the rate begins to drop sharply and the polymerbeads become less porous as the free monomer is absorbed into thepolymer. Such change in the particle character, both in terms ofporosity and particle size distribution, is disadvantageous to themanufacturer in terms of reduced performance and economy of production.Thus, the crenulated porous surface is desired for enhanced plasticizeruptake to form dry blends for various extruding or calendaringoperations. In order to avoid such adverse effects, the manufacturerwill terminate the polymerization reaction prior to complete monomerconversion. The experience of the manufacturer will best determine thepoint at which polymerization is terminated to give high qualitypolymer, although 70 to 90% conversion reflects a general terminationpoint.

Various techniques have been adopted for terminating polymerization. Apurely mechanical approach has involved discharging the polymer slurryinto an evacuated stripper tank and quickly reducing the temperature andpressure to effectively stop polymerization.

Various chemical approaches have also been adopted. The numerousconventional chemical means for terminating vinyl polymerizationreactions are detailed in chemical texts and publications. More recentapproaches have involved adding terminating agents or chain terminatorsto halt free radical propagation, removing unreacted monomer andprocessing the converted slurry to obtain the dry polyvinyl chloride.Thus, compounds such as α-methyl styrene, bisphenol A and varioushindered phenol antioxidants have been added to the reactant mix at apre-determined point for purposes of chain termination. Among thesecompounds, 2,6-di-tert-butyl-4-methylphenol, i.e. BHT, has been mostfrequently utilized as a chain terminator by addition to thepolymerization system at the desired termination point. Various otherhindered phenols having tertiary butyl substituents in the 2- and6-positions on the benzene ring have also been used for this purpose.The performance results have, however, been less than desirable withthese hindered phenols, these materials having little effect when addedin concentrations up to about 250 ppm. BHT has also exhibited thislimitation in being unable to provide effective chain termination at thelower, more desirable concentration ranges.

A further PVC chain termination composition based on hindered phenols isdisclosed in U.S. Pat. No. 4,229,598. This composition is prepared bythe alkylation of a fractionation cut from the product of vapor phasemethylation of phenol. The fraction that is alkylated is a specificmixture of cresylic acids resulting in a corresponding mixture of simplemethyl-t-butyl phenols. These mixtures are necessarily prepared,however, by rather complex and demanding procedures.

It is therefore the primary object of this invention to modify vinylchloride suspension polymerization reactions so as to increase theefficiency of the chain termination step.

It is another object to define a class of additives which when added tothe polymerization system provides the above noted increasedeffectiveness.

It is still another object to define the parameters of this improvedpolymerization procedure.

Various other objects and advantages of this invention will be readilyapparent from the following detailed description thereof.

It has been surprisingly found that effectiveness of chain terminationin vinyl chloride suspension polymerizations can be significantlyincreased by the addition of a class of hindered phenols having a methylgroup on or associated with the phenyl ring, and preferably in theortho- or meta-position on the ring, to the polymerization reactionsystem at a point in time when a designated amount of monomer has beenconverted. Thus, the presence of the methyl group, preferably as asubstitute for one of the tertiary butyl groups generally shielding thephenolic group in the prior art materials, results in an immediatetermination of the polymerization reaction. In this manner, rapid chaintermination is effected thereby facilitating the production of highquality polyvinyl chloride in high yields. These results are achievedwithout any significant modification of the suspension polymerizationreaction and without any concurrent adverse effects. These chainterminators perform substantially better than the prior art materials inproviding rapid and total chain termination and in providing theseimproved performance characteristics at significantly reducedconcentration levels. Since these phenolic materials are recognizedantioxidants and heat stabilizers for polymeric systems, their additionto the polymerization reaction effectively stops free radicalpropagation while correspondingly providing high levels of stability tothe resulting polyvinyl chloride. In addition, approximately 90-95% ofthe chain stopper remains in the resin after stripping and/or drying,thereby effectively eliminating contamination of the recovered monomer.In contrast, lower molecular weight materials such as BHT can beexpected to come off with the excess monomer in the gaseous stream so asto contaminate the monomer and therefore inhibit any polymerizationreaction wherein the recycled monomer is utilized.

The chain terminators applicable for use in the process of thisinvention correspond to the formulae

    RX--(C.sub.a H.sub.2a)-Q                                   (I)

wherein

R is ##STR1##

X is oxygen or sulfur,

a is an integer from 6 to 30,

x is an integer from 0 to 6,

R₁ is alkyl of from 1 to 8 carbon atoms,

Q is hydrogen or --A--(C_(y) H_(2y))--R₂,

A is oxygen, sulfur, ##STR2##

Y is an integer from 0 to 20,

B is lower alkyl or lower alkanoyl,

R₂ is hydrogen, hydroxy, lower alkanoyloxy or ##STR3## and

R₃, R₄, R₅ and R₆ independently are alkyl from 1 to 8 carbon atoms;##STR4## wherein R and X are as previously defined,

b is an integer from 2 to 6,

d is an integer from 3 to 40

Y is oxygen or sulfur, and

R₇ is hydrogen, lower alkyl or ##STR5## with R₅, R₆ and x being aspreviously defined; ##STR6##

wherein R₈ and R₉ independently are hydrogen or lower alkyl, and R_(1O)is lower alkyl.

Compounds of formula I which are preferred exhibit X as oxygen, x as 0to 2, R₁ as alkyl of from 1 to 4 carbon atoms, A as oxygen, y as 0 to 2,R₂ as hydrogen or the indicated phenol and R₅ and R₆ as alkyl of from 1to 4 carbon atoms. Particularly preferred are those compounds wherein R₁and R₆ are tert. butyl positioned in the ortho-position, R₅ is methyland x is 2. Specific compounds of preference are octadecyl3-(3'-methyl-5'- tert.butyl-4'-hydroxyphenyl)propionate and1,6-hexamethylene bis-(3-methyl-5-tert.butyl-4-hydroxy-hydrocinnamate).

Compounds of formula II which are preferred exhibit X and Y as oxygen, xas 0 to 2, R₁ as alkyl of from 1 to 4 carbon atoms, b as 2, d as 3 to20, R₇ as the indicated phenol and R₅ and R₆ as alkyl of from 1 to 4carbon atoms. Particularly preferred are those compounds wherein R₁ andR₆ are tert.butyl positioned in the ortho-position and R₅ is methyl. Aspecific compound of preference is triethylene glycolbis-(3-methyl-5-tert.butyl-4-hydroxyphenyl) propionate.

Compounds of formula V which are preferred exhibit R₈ and R₉ as hydrogenor methyl and R₁₀ as methyl or tertiary butyl. Specific compounds ofpreference are bis-(2-hydroxy-3-tert.butyl-5-methylphenyl) methane and1,1-bis-(2-hydroxy-3,5-ditert.butylphenyl) ethane.

Methods for preparing these hindered phenols are well known to thoseskilled in the art. Specific reference is made to U.S. Pat. Nos.3,285,855, 3,944,594 and 4,032,562 for information regarding the phenolsof formulae I and II and their methods of preparation. In general, theyare prepared from the appropriate acids, acid chlorides or lower alkylesters and alcohols or thio alcohols utilizing well-known esterificationor transesterification methods. The compound of formula III is TOPANOLCA available from ICI Corp., while the compound of formula IV is CYANOX1790 available from American Cyanamid Co. Compounds falling withinformula V include CYANOX 2246 from American Cyanamid Co. and ISONOX 129from Schenectady Chemical.

As previously noted, these hindered phenols are known for theirantioxidant activity in a variety of polymeric substances includingpolyvinyl chloride. When added to such polymers for purposes of thisarea of utility, they are generally added to the final polymericproduct.

Vinyl chloride suspension polymerization techniques are likewisewell-known to those skilled in the art. Such techniques are described indetail in most basic polymer chemistry texts such, for example, as W.Sorenson, "Preparative Methods of Polymer Chemistry", Second Ed.,Interscience Publishers, N.Y. (1968). In general, the reactor is chargedwith the appropriate amounts of suspending agent, initiator andemulsifier in the aqueous reaction system. Typical suspending agentsinclude methyl or ethyl cellulose, gelatin, polyvinyl alcohol or otherwater soluble polymers, while typical initiators includeperoxydicarbonate, benzoyl peroxide, lauroyl peroxide,t-butylperbenzoate, acetyl peroxide, cumene hydroperoxide, di-t-butylperoxide, and the like. The emulsifier is generally added to enhance theporosity of the polymer particles. This charge is generally added to thereactor at ambient temperature. The vinyl chloride monomer is thenintroduced, agitation is initiated and the reactor is heated to thepolymerization range of from 45 to 60° C. The pressure will be in therange of from about 140-150 psig for the 5-7 hour reaction periodnecessary to achieve about 70% monomer conversion, with the residualmonomer being recovered for possible future use. Filtration, washing anddrying complete the polymerization procedure.

In addition to the chemical structure of the instant chain terminators,effective termination is dependent upon the amount of compound added.For purposes of this invention, 25-5000 ppm of hindered phenol, based onthe weight of the vinyl chloride monomer, added at the time of 0 to 95%monomer conversion will provide the desired benefits. Preferred valuesare 50-1000 ppm of hindered phenol added at 70 to 90% monomerconversion, while particularly preferred concentrations are 50-500 ppmof hindered phenol.

It is to be noted that the latter 70-90% range is the desired range forobtaining commercially acceptable yields of polymer. However, theinstant hindered phenols will terminate the polymerization reaction atany stage thereof. Accordingly, the 0-95% range reflects the possibilityof premature termination where emergency conditions, power outages, andthe like, demand such early termination.

The rate of polymerization can be designated in terms of the time to thedesignated amount of conversion of the polymerization reaction and/orthe amount of pressure drop after conversion, the latter beingespecially indicative of termination capability. In this context,"conversion" is designated as the point during polymerization when vinylchloride monomer is no longer available as a free monomer and isabsorbed into the polymer.

The compounds designated herein are seen to provide immediatetermination of vinyl chloride polymerization reactions. This immediatetermination allows for the production of high yields of high qualitypolyvinyl chloride. These compounds are also seen to provide such rapidtermination at concentration levels substantially below that requiredfor currently utilized materials. The compounds simultaneously provideantioxidant properties to the resulting polymeric products. Finally, theresidual monomer is virtually uncontaminated by these compounds.

The following examples illustrate the preferred embodiments of theinvention. In these examples, all parts given are by weight unlessotherwise specified.

EXAMPLE 1

This example illustrates a typical suspension polymerization procedure.

    ______________________________________                                        REAGENT               PARTS                                                   ______________________________________                                        Vinyl Chloride Monomer (VCM)                                                                        100.00                                                  Initiator - Peroxydicarbonate                                                                       0.04                                                    Suspending agent - Methocel type                                                                    0.03                                                    NaOH                  0.10                                                    H.sub.2 O (Deaereated Distilled)                                                                    200.00                                                  Antioxidant           varies                                                  Sodium Lauryl Sulfate 0.02                                                    ______________________________________                                    

A. Filling of Lecture Bottle with 250 g VCM

1. Evacuate air from lecture bottle and weigh.

2. Cool in freezer.

3. Place 1 gallon stainless steel cylinder filled with VCM in hot waterbath (hot tap water).

4. Connect 1 gallon cylinder to 500 ml lecture bottle with flexiblestainless steel hose and connectors.

5. Weigh 500 cc cylinder during addition and stop when approximateweight VCM registers on balance.

6. Weight 500 cc cylinder with no connections to determine exact amountof VCM inside.

B. Charging of Reactor

1. Carefully put thistle tube into reactor's entry port.

2. Add sodium lauryl sulfate through thistle tube.

3. Wash tube with 50 parts water.

4. Add suspending agent in 1% solution (aqueous).

5. Wash tube with 50 parts water.

6. Add NaOH in 1% solution (aqueous).

7. Wash tube with 50 parts water.

8. Add initiator in 7.5% solution (toluene).

9. Wash tube with rest of water.

10. Turn on pressure and temperature recorders.

C. Charging of Reactor with VCM

1. Heat 500 ml lecture bottle in hot water bath (hot tap water).

2. Connect lecture bottle through quick-connect fittings to reactor.

3. Open valves and VCM will flow in within 60 seconds.

4. Close off valves, start stirring motor at 500 rpm and start preheated(62° C.) circulating water bath through reactor outer jacket.

D. Reaction Notes

1. Agitate at 500 rpm and heat to reaction temperature of 57° C. within30 minutes.

2. Typical reaction with peroxydicarbonate initiator achieved 57° C.reaction temperature and 140-150 psi reaction pressure approximately 30minutes after initiating heating. The pressure remains in this range forapproximately 5-7 hours or to about 70% conversion after which thepressure gradually drops.

E. Addition of Chain Stopper at 70% Conversion

1. Add desired amount chain stopper to a 40 cc lecture bottle.

2. Fill 40 cc lecture bottle with 20-30 g VCM as in Procedure A.

3. At 70% conversion connect lecture bottle to addition part of reactorand heat to 90-100° C. with electric heat gun (check with pyrometer).

4. Open valves and VCM/chain stopper solution will flow in.

5. Close valves and reweigh lecture bottle to be sure all VCM/chainstopper solution went into reactor.

F. PVC Recovery

1. Vent unreacted VCM slowly into hood, opening and closing valve untilno pressure is left in reactor.

2. When all VCM is vented, PVC discharged through bottom port intocollection bottle.

3. Filter, then wash with 1000 ml distilled water.

4. Strip wet cake.

The following compounds were utilized in the polymerization proceduresnoted in the following examples.

    ______________________________________                                        Com-                                                                          pound                                                                         ______________________________________                                        (A)                                                                                  ##STR7##                                                                 (B)                                                                                ##STR8##                                                                 (C)                                                                                ##STR9##                                                                 (D)                                                                                ##STR10##                                                                (E)                                                                                ##STR11##                                                                (F)                                                                                ##STR12##                                                                (G)                                                                                ##STR13##                                                                (1) di-t.butyl version of A                                                 (2)   di-t.butyl version of B                                                 (3)   di-t.butyl version of C                                                   (4)                                                                                ##STR14##                                                                (5)                                                                                ##STR15##                                                                (6)                                                                                ##STR16##                                                              ______________________________________                                    

EXAMPLE 2

This example compares the termination effectiveness of a compound of theinstant invention with that of a commercial compound presently beingutilized as a chain terminator in polyvinyl chloride suspensionpolymerizations.

The polymerization procedure of Example 1 was utilized herein. In eachreaction, the pressure after the terminator had been added and thepressure after a 15 hour reaction sequence were noted. These values weredetermined by continuous monitoring of the pressure in the reactor.Small pressure drops after the addition of the terminator are indicativeof effective termination. The following results were obtained:

    ______________________________________                                                             Pressure* Pressure**                                             Concentration                                                                              Drop after                                                                              Drop after 70%                                 Compound                                                                              (ppm)        15 hrs.(psi)                                                                            Conversion (psi)                               ______________________________________                                        A       100          22        16                                                     250           0         0                                             6 (BHT) 250          18        15                                                     500          18        10                                                     1000          5        <3                                             ______________________________________                                         *Initial pressure minus the pressure at 15 hours.                             **Pressure at 70% conversion minus the pressure at 15 hours.             

Although the above noted data indicate effective chain termination byBHT at 1000 ppm, it is important to recognize that they likewiseillustrate a further beneficial performance characteristic of thecompound of this invention, namely, the ability to substantiallyduplicate the chain termination performance of commercial materials atconcentration levels substantially below 1000 ppm.

EXAMPLE 3

The polymerization procedure of Example 1 and the monitoring procedureof Example 2 were again repeated with the compounds noted hereinbelowwith the exception that the compounds were added at the initiation ofthe polymerization reaction. Although this approach does not reflect acommercially desirable procedure, it is satisfactory as a screeningprocedure in order to predict chain termination activity.

Thus, since it can be postulated that the rate of initiation in thepolymerization reaction is related to the rate of termination, anadditive which slows the rate of polymerization also effects the rate oftermination. Accordingly, longer periods to pressure drop are indicativeof more effective chain stoppers. Correspondingly, since pressure dropafter conversion is indicative of continued reaction and a resultantadverse effect on the polyvinyl chloride, lower pressure drops arelikewise indicative of more effective chain stoppers.

The resulting data are noted in the following table.

    ______________________________________                                                            Time to     Pressure Drop                                         Concentration                                                                             Pressure Drop                                                                             after 900 min.                                Compound                                                                              (ppm)       (min.)      (psi)                                         ______________________________________                                        --      --          365         60                                            A       100         540         24                                                    250         >900        <6                                            1       100         450         33                                                    250         465         27                                            B       100         480         30                                            2       100         380         46                                                    250         380         41                                            C       100         570         22                                            3       250         405         36                                            4       100         375         50                                                    250         360         41                                            5       250         375         39                                            6       250         420         21                                                    500         >900        <6                                            ______________________________________                                    

When viewed in the context of the above noted discussion, these dataclearly indicate that the chain terminators of the instant invention aresurprisingly more effective in terminating the polymerization reactionthan the corresponding hindered phenols having tertiary butyl groups inboth ortho positions.

EXAMPLE 4

The following additional compounds reflecting hindered phenols of theinstant invention and commercially available chain stoppers weresubjected to the polymerization and monitoring procedures of Examples 1and 2.

    ______________________________________                                                    Concen-  Pressure Drop                                                                             Pressure Drop                                            tration  after 15 hrs.                                                                             after 70% Con-                               Compound    (ppm)    (psi)       version (psi)                                ______________________________________                                        D           250      8           2                                            E           250      6           0                                            F           250      15          0                                            G           250      7           0                                            α-methyl styrene                                                                    250      6           0                                            Bisphenol A 250      31          26                                           α-methyl styrene/6                                                                  290/570  0           0                                            2           250      35          30                                           --          --       45          45                                           ______________________________________                                    

As noted, the chain stoppers falling within the instant inventionexhibit excellent performance characteristics.

EXAMPLE 5

This example illustrates a further benefit of the chain terminatingagents of this invention as contrasted with a commercial product.

In order to determine the level of contamination of chain termination inthe unreacted vinyl chloride monomer, several of the polyvinyl chlorideproducts prepared in Examples 2 and 3 were analyzed for their terminatorcontent by a gel permeation chromatographic technique. The followingresults were obtained:

    ______________________________________                                                    Initial Conc. of                                                                           Terminator Conc.                                     Compound    Terminator (ppm)                                                                           in PVC (ppm)                                         ______________________________________                                        A           100           95                                                              250          245                                                  2           250          235                                                  6           500          250                                                              1000         435                                                  ______________________________________                                    

In contrasting terminator A of the instant invention with compound 6(BHT), it is seen that a wide disparity exists in the amount ofterminator retained in the polymer. Thus virtually all of A is found inthe polymer, thereby providing virtually uncontaminated residual vinylchloride monomer. In contrast, only about half of the BHT content isretained in the polymer, thereby suggesting the undesirable presence ofsignificant amounts of BHT in the residual monomer.

In summary, this invention provides novel chain terminating agents foruse in suspension polymerization of vinyl chloride monomer. Variationsmay be made in procedures, proportions and materials without departingfrom the scope of the invention as defined by the following claims.

What is claimed is:
 1. In the process for the suspension polymerizationof vinyl chloride monomer which comprises adding the monomer to anaqueous reaction system containing effective amounts of a suspendingagent and a polymerization initiator, terminating the polymerizationreaction and isolating the polyvinyl chloride, the improvement whichcomprises terminating the polymerization reaction by the addition to thereaction system at a point within the range of 70 to 95% monomerconversion of 25-5000 ppm by weight of the vinyl chloride monomer, of acompound corresponding to the formula ##STR17## wherein R is ##STR18## Xis oxygen, b is an integer from 2 to 6,x is an integer from 0 to 6, d isan integer from 3 to 40, Y is oxygen, R₁ is alkyl from 1 to 8 carbonatoms, R₇ is hydrogen, lower alkyl or ##STR19## R₅ and R₆ independentlyare alkyl from 1 to 8 carbon atoms.
 2. The process of claim 1, whereinsaid compound corresponds to formula II and wherein X and Y are oxygen,x is 0 to 2, R₁ is alkyl from 1 to 4 carbon atoms, b is 2, d is 3 to 20,R₇ is ##STR20## and R₅ and R₆ are alkyl of from 1 to 4 carbon atoms. 3.The process of claim 2, wherein R₁ and R₆ are tert.butyl positioned inthe ortho-position and R₅ is methyl.
 4. The process of claim 3, whereinsaid compound is triethylene glycolbis-(3-methyl-5-tert.butyl-4-hydroxyphenyl) propionate.
 5. The processof claim 1, wherein said addition occurs within the range of 70-90%monomer conversion.
 6. The process of claim 1, wherein said additionoccurs at about 70% monomer conversion.
 7. The process of claim 6,wherein 50-1000 ppm of said compound are added to the reaction system.8. The process of claim 7, wherein 50-500 ppm of said compound are addedto the reaction system.
 9. The process of claim 1, wherein 100-250 ppmof triethylene glycol bis-(3-methyl-5-tert.butyl-4-hydroxyphenyl)propionate are added to the reaction system at about 70% monomerconversion.